Three-phase synchronous motors not having a starting cage are tied to the frequency of the three-phase supply network on account of their principle of operation. A start-up, in other words powering up such motors is therefore not possible directly on the network. Rather, a device which enables powering up in the first place is necessary between the three-phase network and the synchronous machine. To this end, a frequency converter is normally used. The frequency converter consists of a rectifier, an intermediate circuit capacitor and an inverter. It is used in order to generate a three-phase current having a selectable frequency.